U-PB GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY OF ~1.1 GA DIABASE DIKES AND SILLS IN THE SOUTHWEST U.S.: IMPLICATIONS FOR RODINIA RECONSTRUCTIONS

The widespread Mesoproterozoic (ca. 1.1 Ga) Southwestern U.S. Diabase Province consisting of dikes, sills, and sheets, is located in southwestern Laurentia. Southwestern U.S. diabase has low to moderate whole rock Mg# (24–64) and concentrations of CaO and Al₂O₃ that suggest fractionation of olivine and plagioclase occurred before emplacement. Total alkali concentrations (Na₂O + K₂O) ranging from 2–5 wt.% indicate that diabase is transitional from tholeiitic to alkaline. Enrichments of large ion lithophile elements relative to mid-ocean ridge basalts and negative Nb anomalies relative to ocean island basalts suggest that diabase was derived from a weakly enriched mantle source. This is supported by whole rock ε_Nd values of 3.2 to -0.5 that correlate with ⁸⁷Sr/⁸⁶Sr_i ratios for whole rock and single crystal plagioclases ranging from 0.694 – 0.710. Some plagioclase crystals have higher ⁸⁷Sr/⁸⁶Sr_i ratios than the corresponding whole rock samples, indicating assimilation of Proterozoic country rock occurred. Xenocrystic zircon in some samples provides further evidence for assimilation. We obtained a U-Pb ID-TIMS age on baddeleyite (ZrO₂) from a diabase dike in the Pinaleño Mountains, Arizona, of 1087 ± 6 Ma (2σ), using the weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 6 fractions. U-Pb SHRIMP dating of zircon from a diabase sill in the Salt River Canyon, Arizona yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1088 ± 11 Ma (2σ; 7 spots). Northwest-striking dikes in Arizona indicate extension occurred perpendicular to Grenville shortening. However, structural and geochemical relationships suggest a mantle plume created mantle-derived magmas that intruded into a faulted upper crust. The age of the Southwestern U.S. Diabase Province is similar to that of the Warakurna Large Igneous Province in Australia (~1075 Ma), supporting a close fit between Laurentia and Australia at this time. The projected intersection of the NW-SE trending dike swarm of the Diabase Province with the western margin of Laurentia may represent a piercing point that constrains the position of Australia along western Laurentia in the Rodinia supercontinent.